Sensors are increasingly important in any field where finer and ever more intelligent control is needed. Examples are found in the growing fields of automotive applications or Wireless Sensor Networks (WSN). In the automotive industry sensors are essential for applications ranging from increased safety to road stability as well as to improve car performance and reliability demanded by customers. Further, compact and low-power sensor interfaces are needed to be competitive on the growing market and to enable new applications for ‘the Internet of things’.
Although the market asks for additional functionality, the price pressure remains. The silicon area is a main contributor to the cost of the sensor interface, therefore the interface has to be made as small as possible. This should not only be valid for the technology nodes that are used today (and which are still relative big for the automotive industry), but also in more advanced technologies.
To realize small-area and low-power constraints, new sensor interface architectures are being investigated. Whereas traditional structures contain large and power-hungry analog building blocks, recently the focus has shifted to frequency conversion instead of voltage conversion. Both approaches are briefly introduced now.
The sensor signal is continuous in time and amplitude. Conventionally this analog signal is amplified, sampled and converted to the digital domain by an analog to digital converter (ADC). A well-known ADC type is a Delta-Sigma ADC, which exploits an oversampling of the input signal and a noise shaping technique to obtain an improved precision. In most applications, the sensor signal frequency varies from DC up to 10-100 kHz, which allows the oversampling needed for a Delta-Sigma converter.
Time/frequency-based conversion mechanisms quantize the continuous input signal by using a known time/frequency signal as a reference instead of a voltage. Typically, a time/frequency-based conversion circuit contains two building blocks: a Voltage-to-Time Converter (VTC) transforms the analog signal c(t) into time or frequency information f(t), while a Time-to-Digital Converter (TDC) digitizes this information with the help of a reference frequency. In order to achieve a desired resolution, an accurate reference clock signal is needed. This time/frequency conversion technique is gaining popularity due to its compatibility with newer CMOS technologies. The resolution now depends on the clock frequency instead of the accuracy of analog voltage values. The reduced gate capacitances result in smaller gate delays, improving the timing resolution in these scaled technologies. Furthermore, when the information is stored as frequency information, it is less prone to noise as opposed to when it is stored as voltage information. Sensor signals are in most applications characterized by their low frequency and therefore ideal to use this way of digitization.
Closed-loop oscillator-based sensor interfaces like in FIG. 1 combine the advantages of time based converters (small and scaling with technology) and sigma-delta ADCs (high accuracy due to oversampling and noise shaping). This architecture is basically a Phase-Locked Loop (PLL) structure, but it has also similarities with a sigma-delta converter as explained in the paper “A novel PLL-based sensor interface for resistive pressure sensors” (H. Danneels et al., Procedia Engineering, Eurosensors 2010, vol. 5, 2010, pp. 62-65). It has the same noise-shaping properties which contribute in increasing the accuracy (expressed e.g. in terms of SNR).
A typical set-up of a closed-loop oscillator-based sensor read-out circuit contains two controlled oscillators (21, 22), for example two voltage controlled oscillators (VCOs) which are matched, a phase detector (3) to compare the two oscillator outputs (41, 42) and a feedback mechanism (4) towards the sensing means (1). The digital output signal (31) of the interface circuit is also derived from the phase detector output.
A conventional oscillator based sensor interface circuit used in a closed loop as illustrated in FIG. 1 operates as follows. A physical quantity (100) is converted by the sensor (1) into electrical signals (11, 12), that influence the oscillators (21, 22) connected to it. The phase of the oscillator outputs (41, 42) is compared in the phase detector (3). The phase detector output signal (31) is fed back through a feedback element (4) to the sensor (1) in order to keep the phase difference between both oscillators small. The closed-loop ensures that the averaged phase detector output (31) is a digital representation of the physical quantity (100). The input signal containing the physical quantity (100) to be converted in the sensor typically represents a pressure, temperature or magnetic field. Also other types of physical signals can be used as input to the interface.
In any application scheme there is always some mismatch between two or more functional blocks of the sensor system, even if they have exactly the same schematic and layout. This also holds for Voltage Controlled Oscillators (or other controlled oscillators, like Current Controlled Oscillators or Sensor Controlled Oscillators). Probably it is even worse for oscillators than e.g. for amplifiers because they continuously toggle and they can so more easily interfere with each other. This means that the oscillators need to be isolated from each other, which can degrade the matching between them. The mismatch also varies over time and over temperature.
If controlled oscillators are used as sensor interface, the mismatch between the oscillators may result in errors, which may affect the signal conversion. Examples are offset, gain and non-linearity. In the paper “Digital-Domain Chopping Technique for PLL-based Sensor Interfaces” (J. Marin et al., Procedia Engineering, Eurosensors 2015, vol. 120, 2015, pp. 507-510) a chopping technique is applied to compensate for the offset error. Additionally calibration can be used to compensate for a big part the initial errors, but it does not compensate the drift over life.
The impact of the oscillator errors on the sensor read-out is also influenced by the common mode of the input of the controlled oscillators. This also causes drift of the sensor interface output. It can be compensated by keeping the common mode signal fixed, but this typically requires an accurate common mode feedback.
These errors and even more a combination thereof limit the accuracy of time-based sensor interfaces.
Consequently, there is a need for an oscillator-based sensor interface wherein at least one or more of these limitations are avoided or overcome.